1. Field of the Invention
The present invention relates to a motor speed controlling device for controlling the rotational speed of a motor in accordance with pulse trains produced from a photocoupler attached to a motor shaft.
2. Description of the Prior Art
There has been known a motor speed controlling device to control a motor to rotate at a target speed. To this end, the rotational speed of the motor is being monitored with a photocoupler which produces pulse trains depending on the rotational speed of the motor. The photocoupler includes an encoder disk formed with a plurality of equi-pitch slits in the periphery thereof. The encoder disk is attached to the driving shaft of the motor to be rotatable therewith. Light emitting diode is disposed at one side of the encoder disk to emit light toward the periphery of the encoder disk, and a photosensor is disposed at opposite side of the encoder disk to receive the light passed through the slit of the encoder disk. The photosensor produces pulse trains whose high-level duration is determined by the rotational speed of the motor. The slits are formed in the periphery of the encoder disk so that a duty ratio of the pulse trains is set to a fixed value, typically 50%. The duty ratio is herein defined by a ratio of a high level duration to one period of the pulse trains. In the case of 50% reference duty ratio, ON and OFF durations of the pulse trains are equal to each other at the time when the motor is rotating at a constant speed. ON duration is defined by a duration from the rise time to the subsequent fall time of the pulse whereas OFF duration is defined by a duration from the fall time to the subsequent rise time of the pulse. The rotational speed of the motor can therefore be detected from either one of the ON and OFF durations of the pulse trains for the 50% reference duty ratio. Based on the detected motor speed, the motor is controlled to rotate at the target speed by feeding back a deviation between the target speed and the actual speed.
However, despite the fact that the pulse duty ratio is set to be 50%, an actual duty ratio tends to shift from the reference duty ratio. Because there are cases that the positional relationship between motor drive shaft and encoder disk or between encoder disk, light emitting diode and photosensor becomes offset due to variations of components, fabricating error between the associated components, and/or aging of the components.
Due to the various reasons set forth above, the actual duty ratio may be shifted from 50% to, for example, 55% or 45 %. The shift of the duty ratio results in detection error of actual rotational speed of the motor. Specifically, when the actual duty ratio is shifted to 55% from 50%, the rotational speed of the motor computed based on the ON duration is slower than the actual rotational speed whereas the rotational speed of the motor computed based on the OFF duration is higher than the actual rotational speed. As a result, the rotational speed of the motor cannot be controlled adequately.